Continuous monitoring of analytes of a patient generally uses an analyte sensor that that is at least partially implanted in the patient so as to be in fluid contact with the patient's analytes such as interstitial fluid or blood. The analyte sensor typically is replaced after a predetermined time period such as three, five or seven day period, when a new sensor is implanted in the patient to replace the old sensor. During the sensor replacement process, a gap or interruption in the analyte monitoring occurs. For example, during the time period in which the patient removes the implanted analyte sensor to replace with a new analyte sensor, the patient is unable to monitor or determine the analyte values such as glucose levels. In this manner, with continuous glucose monitoring systems presently available which use short term analyte sensors, there is always a gap in service during which data associated with the measurement of the patient's analyte levels cannot be obtained.
In addition, calibration of each implanted analyte sensor, which is necessary before data from the analyte sensor can be obtained, is laborious, time consuming, and error prone. Factory calibration is not a practical approach due to substantial sensor to sensor variability of signal strength introduced during the manufacturing process, and also, due to additional variability imposed by the sensors' response to the in-vivo environment which varies from patient to patient.
Thus, typically it is necessary to perform in-vivo calibration, in which the analyte sensor is calibrated, post implantation, by comparison with a reference blood glucose value. Generally these reference blood glucose values include capillary blood glucose values obtained by finger or arm stick using a conventional blood glucose meter. To perform the calibration using the reference blood glucose values, a substantial number of capillary values such as, for example, one to four capillary measurements daily, are necessary to ensure the continued calibration (and thus, accurate) values determined by the analyte sensors.
Moreover, calibrations may sometimes be inaccurate due to transient sensitivity changes which generally occur early in the lifetime of an implanted sensor, and sometimes referred to as early sensitivity attenuation, or ESA. If a calibration is assigned to an analyte sensor undergoing a transient change in sensitivity, inaccurate sensor readings or measurements will result at a later point in time, when the sensitivity reverts to its “true” value.
Further, the typical calibration process is performed for each newly implanted glucose sensor. More specifically, with the placement of each glucose sensor, a new set of blood capillary reference values are obtained, and which is the sole basis (or reference) for calibration of that particular sensor during the usage life of the sensor, for example, during a three, five or a seven day period.
In view of the foregoing, it would be desirable to have an approach to provide methods and system for continuous analyte monitoring where no gap in service can be achieved. In addition, it would be desirable to have methods and a system to verify the stability of a newly implanted sensor, before obtaining user-accessible analyte data from the sensor. Furthermore, it would be desirable to have methods and system for continuous analyte monitoring for continuous calibration of analyte sensors and which minimizes the number of necessary fingerstick (or armstick) calibrations of the analyte sensors using glucose meters, and also, to provide alternate reference.